1. Field of the Invention
The invention relates to a method of transmitting data from a plurality of sub-stations to a main station by way of a common channel which is subdivided into time slots the length and synchronization of which being determined by a corresponding time slot distribution of a data transmission signal transmitted by the main station, each sub-station having in each time slot a predetermined probability p to transmit a data packet.
2. Description of the Related Art
A method of this type is known by the name of slotted ALOHA and is described, for example, in the book entitled "Computer Networks" by Andrew S. Tanenbaum, Prentice/Hall International Editions, Chapter 6, p. 253 ff. With this known method a sub-station sends a data packet which has a time length essentially corresponding to the time length of a time slot in a basically random time slot to the main station. Such a communication link to the main station will henceforth be referenced uplink. There is also a communication link from the main station to the substations, which will henceforth be referenced downlink. The carrier frequency used for the downlink differs from the carrier frequency used for the uplink and is continuously monitored by all the active sub-stations. According to the known method the signal received by the main station is amplified and retransmitted, while the retransmitted signal may be received by all the active sub-stations, but is basically intended for only one specific sub-station. The transmitting sub-station too receives the data packet retransmitted by the main station and compares the data packet with its previously transmitted data packet. On the basis of the result of this comparison the sub-station determines whether the previously transmitted data packet has correctly been received and retransmitted by the main station, in which case a new data packet is transmitted, or transmission is stopped if no new data packet is present, or if an error situation occurs in which case the previously transmitted data packet is retransmitted.
A next transmission of a data packet, whether this is again the already transmitted data packet or a new data packet, is effected in a random time slot relative to the previously used time slot, the distance in time between the previously used time slot and the new time slot depending on said probability p.
The most important cause of an error situation is the concurrent transmission by two or more sub-stations. The data packet transmitted by the main station is then a combination of the two data packets received concurrently by the main station, and thus corresponds neither to the one nor to the other data packet, so that both sub-stations are to transmit their data packets once again. It will be obvious that the probability of such a data packet "collision" increases as the sub-stations transmit data packets more intensively, which causes a reduction of the user efficiency (denoted by the term throughput and expressed in the number of data bits received correctly per second via a communication channel divided by the maximum number of data bits that can be transmitted per second via that communication channel) of the communication channel. On the other hand, it will be obvious that when the sub-stations transmit less intensively, the probability or-"colliding" data packets will be lower, it is true, but a reduction of the throughput of the communication channel may then occur as a result of reduced channel employment. The communication channel therefore has an optimum transmit load with a maximum throughput of about 37% (cf. for example, page 256 of above-mentioned publication).
The throughput in practice of the communication channel will generally be lower than this maximum. An important reason for this is that with the known method the probability p is to be relatively small in order to ensure that the system is not "clogged" by messages to be transmitted even in the statistically improbable but not impossible case of the load of the communication channel being temporarily much larger than the average load.